1. Field of the Invention
The invention relates to a process and apparatus for forming a cardboard tube which is calibrated to exhibit dimensional stability with minimal surface roughness.
2. Description of Background and Relevant Materials
It is known to continuously form cardboard tubes by helically winding a plurality of paper or cardboard strips on a machine called a "spiraler".
This type of machine comprises a fixed cylindrical mandrel on which is helically wound one or more strips, the lead-end of all except the first of which are previously glued to the tail-end of another, and which form predetermined angles with the axis of the mandrel. The tube which is formed is rotated and simultaneously translated longitudinally by a conveyor belt which winds around the resultant tube.
Each continuously formed cardboard tube is then cut during its formation into individual tubes, in a cutting or slicing station.
Depending upon the intended applications of the cardboard tubes, an attempt has been made to produce a tube having good geometric properties, i.e., calibrated to be concentric rather than oval, substantial resistance to atmospheric variations and/or a good surface state with minimal feathering or irregularities.
It is very difficult to obtain a perfect adjustment of the spiraler so that the adjacent edges of two spirals are absolutely joined or to obtain tubes directly at the outlet of the spiraler which have adequate characteristics for most applications. In addition to the previously mentioned concerns this is particularly true for winding thin films or sheets. In such cases there is a major concern for minimizing surface roughness and keeping the superficial hardness perfectly uniform and homogeneous so as to avoid marking the films under the pressure exerted thereon during winding.
Although French Pat. Nos. 1,563,024 and its two additions 95,882 and 2,121,993 relate to these concerns, they do not resolve all of these problems. French Pat. No. 1,563,024 and its first addition form the basis of U.S. Pat. No. 3,580,146. These patents are directed to a method of manufacturing a tube wherein a glued band of compressible material of low density, such as felt board, is wound into a rough tube formed in a conventional manner. In the disclosed procedure, compressible material is used for forming the next-to-last or penultimate ply whereas the last ply is formed from a strip of dense material. Afterwards, the surface of the tube is polished and then coated with a thermosetting resin. Finally, after cutting and drying the tube, each cut tube is passed across a heated drawplate in a manner so as to obtain the calibration, by pressing the next-to-last compressible ply, simultaneously with the polymerization of the resinous coating.
According to first addition No. 95,882, the polishing operation can be eliminated by a preparation and application of a particular glue to the last ply; the second addition No. 2,121,993 describes another heat compression means as an improvement over the drawplate described in the original patent and further discusses a possibility of eliminating the external resinous coating by utilizing a last porous ply which is glued onto the penultimate ply by means of a thermosetting resin.
These prior art processes and apparatus give good enough results for most purposes. The winding of magnetic films, for example video tape, however, requires a tube having a permanent shape, an uniform homogeneity of surface hardness, and an exterior surface with minimal roughness which is substantially more demanding than the typical situation.
These films are very thin and are subjected to extremely high pressures during winding. Consequently, such films are marked over a substantial number of turns due to imperfections in any of the previously mentioned characteristics on the surface of conventional tubes used as spools for these tapes.
In order to obtain better results it has become necessary to entirely re-think the problem. Although there exists, for example, a procedure for winding paper strips in an upright position over a plurality of turns, such a process is difficult, long, costly and poses the problem of joining the ends of adjacent strips.
Thus, the research efforts of the inventor has resulted with the discovery of a new process which solves these problems in a unique manner while preserving the advantages of conventional procedures for manufacturing of a spiraler.